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42492-G5
AC Electrokinetic Microscopy of Single Cells
Funding from the ACS-PRF program has allowed for the development of a new scanning probe microscopy approach utilizing AC electrokinetic for real-time non-contact functional imaging of solid/liquid interfaces, including live cells, with nanometer-scale spatial resolution. Dielectrophoresis (DEP) traditionally describes the mobility of polarizable particles in radio-frequency (100 kHz – 10 MHz) AC electric fields. Similar to the forces in optical trapping, DEP interactions are greatest for large field gradients, such as those adjacent to highly curved electrodes. These forces have been integrated into a scanning probe microscope, allowing for ultra-high resolution dielectric characterization of bacterial and mammalian cells. A brief overview of specific accomplishments demonstrated during the grant period in developing and applying dielectrophoretic force microscopy (DEPFM) are summarized below.
1. Noncontact imaging under aqueous conditions (Anal. Chem., 2005). Despite the numerous advantages of noncontact imaging (minimal sample perturbation, absence of adhesive interactions, protection of delicate tips, etc.), efforts to extent established noncontact methods to liquid environments has been quite difficult. The relatively subtle forces typically used for noncontact imaging in air are often overwhelmed by the increasing in cantilever damping and Brownian noise present in aqueous media. Consequently, there exists a real need for the development of new scanning probe methods amenable to noncontact imaging in liquids. DEPFM. In this study, DEPFM measurements were performed on porous silicon samples, as a representative system with known dielectric properties to allow direct comparison between predictions based on topography and experiments. Two independent analysis approaches yielded mutually consistent results that were also in good agreement with theoretical predictions. These collective results confirmed that DEPFM can allow for noncontact imaging tracking surface topography with the lowest point of tip deflection an astounding 70 nm away from the surface using moderate (10-20 Vpp) peak potentials.
2. DEPFM and DEP Spectroscopy of Red Blood Cells (Biophys. J., 2006). In this work, DEPFM was used to probe an immobilized red blood cell, as a particularly challenging system to test the limits of the instrument. DEPFM allowed for noncontact imaging of red blood cells in aqueous conditions with the lowest point of contact ~20 nm away from the cell surface with only a few Å loss in spatial resolution compared to traditional intermittent contact microscopy. More significantly, changing the AC frequency altered change the magnitude (and even sign) of the DEP force. In systems with mobile ions, the frequency-dependence of DEPFM provides direct access to the characteristic RC time-constant for charge motion, which can in turn be correlated to cellular electrophysiology. Whereas traditional AC electrokinetic methods can provide DEP spectra for whole cells and cell ensembles in ~10's of minutes, DEPFM has been shown to yield meaningful DEP spectra in as little as 1/8 of a second. This accomplishment represents a 3-4 order of magnitude reduction in the time-scale required for dielectric characterization with a simultaneous improvement by 3-4 order of magnitude in the spatial resolution over which DEP measurements can be performed.
3. Nanoscale Control Over Electrodynamic Cavitation (J. Vac. Sci. Tech. B, 2008). An unexpected consequence of the DEPFM studies supported by ACS-PRF was the observation of substantial enhancements in the oxidation rates of silicon substrates when using and AC electric field in water. Anodic oxidation of Si using a conducting scanning probe tip as an electrode represents a promising approach for nanolithography, but is limited by the slow rate of oxidation and the relatively small features that can be generated due to kinetic limitations in the oxidation reactions. Interestingly, we observed large-scale structures (>100 nm in height) forming at discrete events in time when a low-frequency (~100 Hz) AC electric field was applied between the tip and the substrate in an aqueous environment. It is hypothesized that nanoscale bubbles are forming upon electrolysis of water, which then reverses upon inversion of the applied potential. Bubble collapse could provide the energetic impetus necessary to overcome the kinetic barriers to silicon oxidation, which is consistent with the observations. It is well-established that bubble collapse in solution during sonication results in localized regions of remarkably high temperature and pressure. However, this study represents the first demonstration to our knowledge of similar effects driven by electrolysis in a controlled manner.
